Re: [PATCH] xen/pcifront: check that an AER callback exists before calling it

[Bjorn Helgaas]

On Mon, Oct 05, 2026 at 10:30:18PM +0900, Yehyeong Lee wrote:
> pcifront_common_process() handles an AER request from the backend by
> dispatching on aer_op.cmd to the bound driver's PCI error handler.  It
> only checks that err_handler and err_handler->error_detected are present,
> then for the mmio_enabled, slot_reset and resume commands it calls the
> corresponding callback unconditionally.  Those three callbacks are
> optional -- the PCI core NULL-checks each of them individually before
> use -- and many drivers (for example igb, igc, ice and ixgbevf) install
> error_detected without all of them.
> 
> aer_op.cmd comes from the shared ring, so a malicious or buggy backend
> can send XEN_PCI_OP_aer_mmio (or _slotreset/_resume) for a device whose
> driver leaves that callback NULL and make the frontend call through a
> NULL pointer, crashing the guest.
> 
> Check each callback before calling it, as the PCI core already does, and
> fall through to PCI_ERS_RESULT_NONE when it is absent.
> 
> Fixes: 956a9202cd12 ("xen-pcifront: Xen PCI frontend driver.")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Yehyeong Lee <yhlee@xxxxxxxxxxxxxxxxxx>

Seems right to me, but I assume the Xen folks will pick this up.

Acked-by: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>

> ---
> There is an in-flight fix for a refcount leak in this same function
> ("xen/pcifront: Fix PCI device reference leak in AER handling"); this
> change is orthogonal and applies in either order.
> 
>  drivers/pci/xen-pcifront.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/pci/xen-pcifront.c b/drivers/pci/xen-pcifront.c
> index cffc32d660327..490913676f244 100644
> --- a/drivers/pci/xen-pcifront.c
> +++ b/drivers/pci/xen-pcifront.c
> @@ -599,12 +599,17 @@ static pci_ers_result_t pcifront_common_process(int cmd,
>  		case XEN_PCI_OP_aer_detected:
>  			return pdrv->err_handler->error_detected(pcidev, state);
>  		case XEN_PCI_OP_aer_mmio:
> -			return pdrv->err_handler->mmio_enabled(pcidev);
> +			if (pdrv->err_handler->mmio_enabled)
> +				return pdrv->err_handler->mmio_enabled(pcidev);
> +			break;
>  		case XEN_PCI_OP_aer_slotreset:
> -			return pdrv->err_handler->slot_reset(pcidev);
> +			if (pdrv->err_handler->slot_reset)
> +				return pdrv->err_handler->slot_reset(pcidev);
> +			break;
>  		case XEN_PCI_OP_aer_resume:
> -			pdrv->err_handler->resume(pcidev);
> -			return PCI_ERS_RESULT_NONE;
> +			if (pdrv->err_handler->resume)
> +				pdrv->err_handler->resume(pcidev);
> +			break;
>  		default:
>  			dev_err(&pdev->xdev->dev,
>  				"bad request in aer recovery operation!\n");
> -- 
> 2.43.0
>